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Old 28th April 2013, 04:04 PM   #131
Magz is offline Magz  United States
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OK, enough theorizing. In the next few days I'll hook up my 2500V PT to a variac and run the HT output to the board. Then I'll run the voltage up as high as I dare and check the drops across my "worst" bridge leg (luckily I noted which one that is). That ought to give us enough data to answer this question. Stay tuned; first I want to clean up the board a bit to ensure the voltages stay where they should (remove adjacent solder pads, round sharp points, maybe some high dielectric conformal coating).

PS: Yes I have a 50kV high voltage probe if I get too high up there ;-)

PPS: I'm having fun!
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Old 28th April 2013, 04:39 PM   #132
nhuwar is offline nhuwar  United States
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Hey Magz,

If you don't mind me asking, how much is monolith charging for the output transformers?

Thanks,
Nick
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Old 28th April 2013, 05:43 PM   #133
tomchr is offline tomchr  United States
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Quote:
Originally Posted by Magz View Post
The 40 diodes I have all were within .007V for Vf - do they still need to be treated with the worst case scenario?
You're missing the point. If you read the SGS app note, you'll find the following in the first paragraph of the conclusion:

Quote:
When using several fast rectifiers in series it is necessary to make sure that no diode will be subjected to continuous or transient voltages in excess of their ratings.
In most cases, this is achieved by using sharing networks across each diode.
It is important to optimize this circuit in order to reduce power consumption and to save space.
(Emphasis mine). Please read through that app note. It states specifically under which circumstances that you can eliminate the sharing networks. Do the math for your scenario rather than relying solely on experimentation and blog opinions.

You are trying to balance the REVERSE voltage, VR. **NOT** the FORWARD voltage, VF. It doesn't matter that the FORWARD voltage of the diodes in your batch are within a few mV of each other. It's the REVERSE voltage that causes the problem. If you exceed the REVERSE voltage across any of the diodes at any point in time, even briefly, it will break down catastrophically. When it does, it will take the remaining diodes with it. With the amounts of energy you are dealing with, this will cause the diodes to EXPLODE. Seriously.

Quote:
Originally Posted by Magz View Post
If you surf the Ham Radio forums the consensus now seems to be that if the diodes are from the same batch, then resistors aren't needed; the caveat is that they are not talking specifically about SiC Schottky diodes, rather they focus on standard Si diodes.
No offense to any HAM out there. I used to be one myself, as a matter of fact. But how many HAMs, honestly, understand the semiconductor physics behind the reverse breakdown of a diode? Do they understand it better than the manufacturer does? I doubt it. Please read that app note.

If you're curious about the semiconductor physics, I suggest reading Ben Streetman, "Solid State Electronic Devices".

I cannot believe you are willing to risk a $10k amp build to save a few bucks on resistors. Never mind risking your life...

~Tom
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Last edited by tomchr; 28th April 2013 at 05:46 PM.
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Old 28th April 2013, 05:58 PM   #134
Magz is offline Magz  United States
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Too much drama. Experimental evidence can't hurt, besides I'm enjoying myself, after all this is a hobby. Who knows, maybe it will help someone else down the road who has this same question come to a faster decision.

PS: Don't anyone else try this, EVER. It's too dangerous. There, that's my legal disclaimer.
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Old 28th April 2013, 06:00 PM   #135
nhuwar is offline nhuwar  United States
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If you read in current arrl amateur radio hand book they say this is not necessary with modern diodes.

I'm not saying don't put snubbers on them. Just food for though is all.

Nick



Quote:
Originally Posted by tomchr View Post
You're missing the point. If you read the SGS app note, you'll find the following in the first paragraph of the conclusion:

(Emphasis mine). Please read through that app note. It states specifically under which circumstances that you can eliminate the sharing networks. Do the math for your scenario rather than relying solely on experimentation and blog opinions.

You are trying to balance the REVERSE voltage, VR. **NOT** the FORWARD voltage, VF. It doesn't matter that the FORWARD voltage of the diodes in your batch are within a few mV of each other. It's the REVERSE voltage that causes the problem. If you exceed the REVERSE voltage across any of the diodes at any point in time, even briefly, it will break down catastrophically. When it does, it will take the remaining diodes with it. With the amounts of energy you are dealing with, this will cause the diodes to EXPLODE. Seriously.



No offense to any HAM out there. I used to be one myself, as a matter of fact. But how many HAMs, honestly, understand the semiconductor physics behind the reverse breakdown of a diode? Do they understand it better than the manufacturer does? I doubt it. Please read that app note.

If you're curious about the semiconductor physics, I suggest reading Ben Streetman, "Solid State Electronic Devices".

I cannot believe you are willing to risk a $10k amp build to save a few bucks on resistors. Never mind risking your life...

~Tom
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Old 28th April 2013, 06:10 PM   #136
Magz is offline Magz  United States
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Quote:
Originally Posted by nhuwar View Post
Hey Magz,

If you don't mind me asking, how much is monolith charging for the output transformers?

Thanks,
Nick
PM me.
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Old 28th April 2013, 09:13 PM   #137
tomchr is offline tomchr  United States
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Good luck with your amp. Stay safe. Stay alive.
__________________
21st Century Maida Regulator, Universal Filament Regulator, Damn Good 300B, Novar Spud, and more...
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right
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Old 28th April 2013, 09:41 PM   #138
nhuwar is offline nhuwar  United States
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Quote:
Originally Posted by Magz View Post
PM me.


Pm sent.
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Old 28th April 2013, 10:16 PM   #139
Magz is offline Magz  United States
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Quote:
Originally Posted by tomchr View Post
Good luck with your amp. Stay safe. Stay alive.
Thank you.
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Old 29th April 2013, 07:57 AM   #140
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Maybe you can get away without, but balancing resistors can help reliability - and suitable parts can be procured to make it easy.

For example, I use the Welwyn MH37 series, which are rated for 3500V, and are almost as cheap as ordinary resistors:

MH37-470KJI - WELWYN - RESISTOR, H/V 0.5W 470K | Farnell United Kingdom

One more subtle reason for using them: Before the diodes break down destructively, you can sometimes get transient breakdown/leakage effects, that may not destroy the diode, but cause a lot of noise.

Especially if you choose to avoid balancing resistors, I would recommend a fuse in the PT secondary winding circuit - rated to be able to manage the HT voltage. A microwave cooker fuse is made for 5kV, and may be useful, depending on the VA rating of the PT:

Microwave Oven High Voltage Fuse 800mA 0.8A 5kV | eBay

A fuse in the secondary gives closer control, since the power-ON surge needs no accounting for.
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